LEDs are formed by growing epitaxial layers, including p-type and n-type layers, on a growth substrate. A light-emitting active layer is sandwiched between the n and p layers. Green, blue, and ultraviolet LEDs are typically gallium-nitride based, where the growth substrate is typically either sapphire (an insulator), SiC (a semiconductor), silicon, SiC-on-insulator (SiCOI), or other engineered substrate. Infrared, red, and amber LEDs are typically some combination of AlInGaPAs and grown on a GaAs or InP substrate. The growth substrate has a lattice structure similar to the lattice structure of the LED material.
It is sometimes desirable to remove the growth substrate to, for example, improve the optical properties of the LED or to gain electrical access to the LED layers. In the case of a sapphire substrate, removal may be by means of laser melting a GaN/sapphire interface. In the case of Si or GaAs substrates, more conventional selective wet etches may be utilized to remove the substrate.
Since the LED epitaxial layers are extremely thin (e.g., less than 10 microns) and delicate, before removing the growth substrate, the LED wafer must first be attached to a support substrate so that the LED layers are sandwiched between the growth substrate and the support substrate. The support substrate has the desired optical, electrical, and thermal characteristics for a particular application of the LED. The growth substrate is then removed by known processes. The resulting wafer with the support substrate and LED layers is then diced, and the LED dice are then mounted in packages.
A package typically includes a thermally conductive plate with electrical conductors running from the die attach region to the package terminals. The p and n layers of the LED are electrically connected to the package conductors. In the case of a vertical injection device, the support substrate is metal bonded to the package, providing a current path to the n or p-type LED layers adjacent to the support substrate, and the opposite conductivity type layers are connected via a wire (e.g., a wire ribbon) to a package contact pad. In the case of a flip-chip LED (n and p-type layers exposed on the same side), both n and p-connections are formed by die attaching to multiple contact pads patterned to mate to the n and p-contact metallizations on the die. No wires are required.
Some drawbacks with the above-described devices are described below.
The support substrate between the LED layers and the package provides some electrical and thermal resistance, which is undesirable. The support substrate itself adds expense and height. The process of attaching the support substrate to the LED wafer is costly, and yield is lowered.
Accordingly, what is needed is a technique to avoid the above-described drawbacks.